1. Field of the Invention
The invention relates to a device for measuring the clearance between the tips of the blades of a ring of blades and the interior wall of the casing (5) surrounding said ring of blades in a turbomachine.
2. Discussion of the Background
In the development of turbomachines, knowledge of the blade tip clearances is a fundamental element in mastering the thermodynamics of the machine, so as to improve its performance.
Every effort is made to make this clearance as small as possible, while at the same time avoiding the risk of the blade tips rubbing against the interior wall of the casing.
However, given that it is extremely difficult to optimize this clearance through calculation, because of the many mechanical or thermal parameters involved, it becomes necessary to resort to measuring the clearance between the blade tips and the interior wall of the casing using sensors.
The clearance may be measured continuously in operation. In this case, use is made of sensors of the capacitive type, which give low-amplitude signals.
The clearance may also be measured spot wise during testing or maintenance. In this case, use is generally made of a probe made of an abrasive material which comprises a U-shaped electrical circuit, the base of which is a certain distance from the internal wall of the casing. When the base of the electrical circuit is eroded by abrasion, the electrical circuit opens, the clearance is then smaller than the distance than corresponds to a calibrated depth of clearance. This sensor has the advantage that it is robust, but it gives just one result during the course of the test.
Now, it is often desirable to measure the occurrence of several sites of clearance during the same turbomachine test. To do this, use is made in practice of several sensors of the resistive type at the periphery of the ring of blades, each of which defines a calibrated depth of clearance.
Starting out from this prior art, the invention set itself the objective of providing provides a simple, reliable and inexpensive device that made it possible to measure several decreasing depths of clearance.
The invention achieves its objective in that the device proposed comprises a probe which can be mounted radially on the casing and at least one end of which is made of a material that can be abraded by the tips of the blades as they rotate, a printed circuit arranged in the mid-plane of said probe which contains the axis of rotation of the ring of blades, this printed circuit comprising a number of adjacent U-shaped electrical circuits the bases of which are arranged in a probe end likely to be abraded by the blade tips and lie at different depths from a reference level defining the interior wall of the casing, and means for recognizing the U-shaped electrical circuits which have been broken by abrasion and the electrical circuits which are intact.
Each base thus corresponds to a calibrated depth of clearance and the device makes it possible, in the course of the one same test, to observe any decrease in clearance, and the position of the blade tips between two calibrated depths. Implementation of the device entails accurately positioning a single sensor, whereas in the prior art, several sensors each having just one U-shaped electrical circuit, had to be positioned accurately.
To reduce the number of conducting tracks in the printed circuit, two adjacent electrical circuits have a common branch.
Advantageously, the depths of the bases increase by a predetermined step between the shortest lateral electrical circuit and the longest lateral electrical circuit.
To reduce the external wiring of the probe, the outer branch of the shortest lateral electrical circuit is connected to a first electrical terminal, and the other branches of the electrical circuits are connected to a second electrical terminal via a resistor of a set of resistors.
As a preference, the resistors of the set all have practically the same resistance. The terminals are connected to an electrical circuit external to the probe which comprises means for measuring the equivalent impedance of the resistors of the intact circuits. This impedance measurement makes it possible to determine the number of longest circuits broken and thereby the position of the clearance remaining between two calibrated depths of clearance.